[GC] Add struct.new and start to test interesting execution (#3433)

With struct.new read/write support, we can start to do interesting
things! This adds a test of creating a struct and seeing that references
behave like references, that is, if we write to the value X refers to, and
if Y refers to the same thing, when reading from Y's value we see the
change as well.

The test is run through all of -O1, which uncovered a minor issue in
Precompute: We can't try to precompute a reference type, as we can't
replace a reference with a value.

Note btw that the test shows the optimizer properly running
CoalesceLocals on reference types, merging two locals.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/wasm.h b/src/wasm.h
index c1936cb46..0ea3ffeb7 100644
--- a/src/wasm.h
+++ b/src/wasm.h
@@ -1345,9 +1345,17 @@ public:
 
 class StructNew : public SpecificExpression<Expression::StructNewId> {
 public:
-  StructNew(MixedArena& allocator) {}
+  StructNew(MixedArena& allocator) : operands(allocator) {}
 
-  void finalize() { WASM_UNREACHABLE("TODO (gc): struct.new"); }
+  Expression* rtt;
+  // A struct.new_with_default has empty operands. This does leave the case of a
+  // struct with no fields ambiguous, but it doesn't make a difference in that
+  // case, and binaryen doesn't guarantee roundtripping binaries anyhow.
+  ExpressionList operands;
+
+  bool isWithDefault() { return operands.empty(); }
+
+  void finalize();
 };
 
 class StructGet : public SpecificExpression<Expression::StructGetId> {